An in-situ on-line detection device and detection method for a long-distance metallurgical liquid metal component. The detection device comprises a front-end high-temperature resistant probe (18), a middle-end optical sensing device (19) and a back-end control platform (24), wherein the head of the front-end high-temperature resistant probe (18) is placed in a liquid metal (22), the tail thereof is coaxially connected to the middle-end optical sensing device (19), and an optical window (15) is arranged in the connection position; and the middle-end optical sensing device (19) is connected to the back-end control platform (24) through a signal line (25). The detection device and detection method can provide a timely and valid message for quality control and a melting end, so that the detection time is greatly shortened, the detection distance can be adjusted extensively, the measurement result is accurate, and it can be achieved to measure components that are difficult to measure, such as C, S, P, etc.